1. Field of the Invention
The present invention is related to a retardation compensation element which is used in combination with a liquid crystal display panel, and a manufacturing method thereof.
2. Description of Related Art
A large number of liquid crystal display panels have been employed in direct viewing type display apparatuses of television receivers and of various appliances. Also, liquid crystal display panels have been employed as image display devices of liquid crystal projectors. Liquid crystal display panels have been constructed in such a manner that a large number of liquid crystal cells are arrayed in a pattern in correspondence with pixel arrays. Various types of liquid crystal display panels are known, depending upon operating modes of liquid crystal molecules sealed in liquid crystal cells, for instance, TN (Twisted Nematic) type LCD panels; VAN (Vertical Alignment Nematic) type LCD panels; IPS (In-Plane Switching) type LCD panes; OCB (Optically Compensatory Bend) type LCD panels, and the like.
As liquid crystal display panels employed in liquid crystal projectors, since such liquid crystal display panels having superior light-shielding characteristics have been properly employed in order to increase contrasts of images on display screens, there is a trend that, for instance, VAN type liquid crystal display panels have been largely employed. In the VAN type liquid crystal display panel, the most of rod-shaped liquid crystal molecules within a liquid crystal layer are orientated substantially perpendicular to substrates under such a no voltage applying condition that a voltage is not applied between the substrates sandwiching the liquid crystal layer, and a better light-shielding characteristic and a higher contrast can be achieved by being combined with one pair of polarizing plates which are arranged in a crossed Nicols arrangement.
On the other hand, as general drawbacks caused by liquid crystal display panels, it is known that viewing angles thereof are narrow. For instance, In the case where the VAN type liquid crystal display panel is brought into the no voltage applying condition so that liquid crystal molecules are vertically orientated, light vertically entering the liquid crystal layer can be sufficiently shielded, but light obliquely entering the liquid crystal molecules is birefringently refracted in response to incident angles thereof, and thus, generally speaking, linear polarization is transformed to elliptical polarization. As a result, a portion of polarized light components passes through a polarizing plate arranged in the crossed Nicols arrangement on the side of a light-emitting plane and then gives an influence on a black level on the display screen becoming bright, so that this polarized light component may lower the contrast. Also, when the crystal liquid molecules within the liquid crystal layer constitute either horizontal orientation or intermediate orientation, it is not possible to avoid that a quality of a displayed image is lowered, depending upon a difference in birefringence caused by angles of light entering the liquid crystal layer.
The above-described drawbacks of the liquid crystal display panels can be improved by employing a retardation compensation element disclosed in e.g., JP-A-2006-91388 and JP-A-2004-102200. Due to the birefringent characteristic of the liquid crystal layer, the liquid crystal layer is operated as a so-called “positive retarder” by which a phase of an ordinary light component of light entering the liquid crystal layer precedes that of an extraordinary light component thereof. However, the retardation compensation element is operated as a so-called “negative retarder” by which the phase of the ordinary light component of the entered light is delayed with respect to the extraordinary light component thereof. As a consequence, since the retardation compensation element is combined with the liquid crystal display panel, the birefringent characteristics thereof may be offset with each other, so that the reduction of the contrast may be suppressed.
As described in JP-A-2006-91388 and JP-A-2004-102200, since high luminance lamps are employed as light sources in the liquid crystal projectors, sufficiently high heat resisting properties are required for the retardation compensation elements. As described in JPA-2006-91388, if a crystal plate of an optical anisotropic material is employed in the retardation compensation element, then a retardation compensation element having a superior heat resisting property can be obtained. However, such a crystalline plate itself is high cost; when the crystal plate is processed, cutting of crystal planes and dimensional precision must he strictly managed, and assembling and adjusting works of the retardation compensation element are cumbersome. In this regard, the retardation compensation element described in JP-A-2004-102200 has the following advantages: That is the retardation compensation element can be constructed of multiple layers and can be manufactured by stacking transparent thin layers (made of inorganic material); the retardation compensation element can have superior heat resisting property and superior durability and further superior mass production applicability; and the retardation compensation element can be provided at low cost.
The retardation compensation element described in JP-A-2004-102200 includes multiple layers manufactured by alternately stacking two types of thin layers on each other, the two types of thin layers having different refractive indices from each other and having such thicknesses that optical interference does not occur when visible light enters the layers, and is operated as a negative uniaxial c-plate in the crystal optics. As the two types of thin layers, various thin layers below may be employed, namely, as thin layers having high refractive indices, TiO2, ZrO2, Nb2O5, and the like may be employed, whereas as thin layers having low refractive indices, SiO2, MgF2, CaF2, and the like may be employed. Also, these thin layers may be manufactured by employing multilayer forming methods such as a vapor deposition method, a sputtering processor, and furthermore, an ion plating method. These thin layers may be simply manufactured by employing, for example, a sputtering apparatus shown in FIG. 7.
FIG. 7 conceptionally shows a sputtering apparatus for manufacturing such a retardation compensation element in which two types of thin layers made of inorganic materials are alternately stacked. In the sputtering apparatus, an exhaust tube 3, a nozzle 4 for conducting discharge gas, and nozzles 5, 5 for conducting reaction gas is communicated with a vacuum chamber 2. Inside the vacuum chamber 2, a drum 6 is rotatably assembled around a vertical supporting shaft, and transparent substrates 7 on which thin layers are formed are supported by an outer circumferential plane of the drum 6. En this figure, such a condition is illustrated that 5 sheets of substrates 7a to 7e are arranged in a longitudinal direction on only one plane of flat outer circumferential planes of the drum 6 having an octagonal cylindrical shape, but when thin layers are actually manufactured, the substrates 7 may be supported on all of the octagonal planes of the drum 6 in a similar manner. Also, if the drum 6 has any other structures capable of supporting the substrates 7a to 7e equidistant from the rotation center of the drum 6, then the shape of the drum 6 may be properly selected from a hexagonal cylindrical shape, a cylindrical shape, and the like. Moreover, a total number of substrates 7 which are supported on the outer circumferential planes of the drum 6 may be properly increased, or decreased in correspondence with sizes of substrates and sizes of drums.
The respective substrates 7a to 7e are fixed on the substrate holder 8 freely rotatable over the outer circumferential planes of the drum 6, and when the substrate holder 8 is rotated, the respective substrates 7a to 7e are rotated by an angle of 90 degrees around the normal of the surface of the substrate 7. The rotation direction may be selected from any of the clockwise direction and the counterclockwise direction. Two target materials 9 and 10 are provided within the vacuum chamber 2 in such a manner that these target materials 9 and 10 are located opposite to the respective substrates 7a to 7e. Since these target materials 9 and 10 include thin-layer materials which are alternately stacked on the surfaces of the substrates 7a to 7e. Nb (niobium) and Si (silicon) is employed as one example. Then, the drum 6 is rotated at a constant speed and a chemical reactive sputtering process with these target materials 9 and 10 is carried out in an oxygen gas atmosphere, so that multiple layers on the substrates 7a to 7e, in which an Nb2O5 layer having a high refractive index (n=2.38) and an SiO2 layer having a low refractive index (n=1.48) are alternately stacked, can be obtained.
If the layer having the high refractive index and the layer having the low refractive index are stacked on each other in such a manner that physical layer thicknesses thereof are made thin, for instance, approximately 10 to 20 nm, then a retardation compensation element (negative retarder) having a birefringence “Δn” may be obtained. A magnitude of the birefringence “Δn” is determined based upon both a difference between the refractive indices of two types of thin layers and a ratio of the physical layer thicknesses of the thin layers; and retardation “dΔn” is determined based upon a product calculated by this birefringence “Δn” and an entire layer thickness “d” of the multilayer layer. As a consequence, thin layers are designed in correspondence with a value of positive retardation “dΔn” which is produced by a liquid crystal layer of a liquid crystal display panel to which the designed thin layers are applied, and thus, both birefringence “Δn” and an entire layer thickness “d” are determined. In order to simplify layer forming steps, it is advantageous to alternately stack two types of thin layers on each other. Alternatively, even if three or more thin layers having different refractive indices from one another are used in combination, an effect of a retardation compensation, similar to that of the two types of thin layers, may be achieved.
FIG. 8 shows the thus-obtained retardation compensation element 20. That is, a retardation compensation layer 21 of a multiple layer in which the high refractive index layer “L1” and the low refractive index layer “L2” are alternately stacked is formed on a front surface of a substrate 7, and this retardation compensation layer 21 is operated as the negative retarder. An antireflection layer may he alternatively provided on a rear plane of the substrate 7, a boundary plane between the substrate 7 and the retardation compensation layer 21, or a front plane of the retardation compensation layer 21. As represented in FIG. 10, this retardation compensation element 20 does not have a birefringent characteristic with respect to light “P1” which vertically enters the retardation compensation layer 21. As a consequence, no phase difference is produced in light emitted from the retardation compensation element 20. This is due to the fact that, for instance, a VAN type liquid crystal layer does not have the birefringent characteristic with respect to light which vertically enters this VAN type liquid crystal layer under no voltage applying condition. However, with respect to light “P2” which enters the retardation compensation element 20 at an incident angle “θ”, even if most of the liquid crystal molecules constitute vertical orientation attitudes under no voltage applying condition, the liquid crystal molecules develop a birefringent characteristic with respect to this oblique incident light “P2”, and thus, may produce positive retardation “dΔn” in response to an optical path length which is determined based upon the incident angle “θ” As a consequence, the retardation compensation element 20 generates negative retardation “dΔn” in response to the incident angle “θ” with respect to the light P2 having the incident angle “θ” so as to compensate the positive retardation occurred in the liquid crystal layer.
FIG. 11 shows an occurrence distribution characteristic of the negative retardation “dΔn” by way of a conoscope type graphic representation, the negative retardation “dΔn” being produced by the retardation compensation element 20 manufactured by the sputtering apparatus shown in FIG. 7 with respect to oblique incident light whose incident angle “θ” is approximately 30 degrees. The value of the negative retardation “dΔn” is equivalent to a length of a radius measured from a center of the graph. As indicated by a characteristic line “Q1”, if the value of the negative retardation “dΔn” is substantially constant irrespective of an azimuth angle (being equivalent to such an angle that light P2 is fixed along constant direction and substrate 7 is rotated around normal line), there is no problem. However, as represented by another characteristic line “Q2”, there are some possibilities that such a retardation compensation element 20 whose retardation value “dΔn” is varied in response to the azimuth angles may be manufactured. In such a retardation compensation element 20, the above-described fact implies that the positive retardation “dΔn” occurred in the liquid crystal layer cannot be compensated, depending upon the direction along which the liquid crystal display panel is observed, and thus, the larger the value of the incident angle “θ” becomes, the greater the degree of the influence thereof is increased. Moreover, the following could be confirmed which may impede that a retardation compensation effect in higher precision is achieved. That is, the retardation compensation element having such a trend as indicated by the characteristic line Q2 with respect to the light P2 may produce negative retardation “dΔn” which exceeds 1 nm even with respect to the vertically entered light P1.
This is specifically caused by a retardation compensation element for obtaining a retardation compensation effect by alternately stacking thin layers having two refractive indices (namely, high and low refractive indexes). It is conceivable that the above problem may be caused by a small difference contained in layer forming conditions of retardation compensation layers in which layer thicknesses of the respective thin layers are made sufficiently thin, and furthermore, entire layer numbers reach several tens of layers, or one hundred and several tens of layers to several hundreds of layers. For example, when a general-purpose optical interference thin layer is formed by employing the sputtering apparatus shown in FIG. 7, as to both a thin layer formed on the substrate 7a supported on a first stage of the drum 6 and another thin layer formed on the substrate 7c supported on a third stage of the drum 6, although optical characteristics of these thin layers are not completely coincident with each other, practically speaking, there is substantially no possibility that this difference of these optical characteristics may cause a problem. In contrast to the optical interference thin layer, in the retardation compensation layer, a large number of the individual thin layers are stacked, changes in physical properties caused by a slight difference contained in the layer forming conditions are accumulated and emphasized. As a result, as can be seen from the characteristic line Q2 of FIG. 11, the retardation distribution characteristic related to the azimuth angles becomes non-uniform, so that superior retardation compensation effects can be hardly obtained.